Sometimes, Paint changes the RGB value of the color of a group of pixels in a PNG image when it gets saved. It only changes a little bit, for example from RGB(128,128,128) to RGB(127,127,127), so you can't see it just by looking at the picture, but you can notice it when using the Fill function for example.
To show you a concrete example of this, here is a picture of a 256x256 square which was originally completely filled with RGB(128,128,128) gray, and some parts have when saving the image become RGB(127,127,127) or RGB(126,126,126). To make the difference visible, I've colored the RGB(128,128,128) in white, the RGB(127,127,127) in gray, and the RGB(126,126,126) in black.
Why does it do this?